The interaction of MHC products and corresponding MHC-recognition structures expressed by thymocytes is probably a major factor in the proliferation, differentiation and selection of MHC-restricted T cells within the thymus. In situ evidence suggests this is the case, since the MHC-restricted antigen receptors patch to areas of contact with Ia+ epithelial cells in the thymic cortex. Definition of the interaction between MHC products and thymocyte recognition structures is crucial to the understanding of T cell differentiation within the thymus. This proposal addresses this interaction in situ and in vitro, comparing young and aged thymuses, where in the later, epithelial Ia expression is reduced and thymus function has declined. In situ immunoelectron microscopic studies will focus on four aspects on the interaction between MHC products and thymocytes: 1) expression of Ia by epithelium and hematogenous cells; 2) expression of recognition molecules by thymocytes, i.e., antigen receptors and L3T4 molecules; 3) simultaneous distribution of these molecules at sites of cell contact; and 4) the consequence of aging on their expression and interaction. The second part of the proposal will examine cultured thymic epithelial cells and their interactions with thymocytes in vitro. After defining epithelial heterogeneity in vitro as it is known to exist in vivo. The following topics will be addressed: 1) expression of epithelial Ia in response to soluble immune factors and non-immune agents; 2) the ability of epithelial cells to interact with thymocytes via MHC recognition structures and the effect of this interaction on epithelial Ia expression, and 3) the comparison of epithelial cells from adolescent and senescent thymuses to respond to Ia-inducing agents and to interact with thymocytes. The in situ and in vitro studies will complement each other, where each approach provides unique information and confirms hypotheses based on evidence derived from the other. Identifying the interaction(s) between thymocytes and thymic epithelial cells with involve MHC molecules will advance our knowledge of the thymic role in cell differentiation and may provide important insight into the process of thymus involution and senescence of T cell function observed in aged laboratory animals and man.